1. Field of the Invention
This invention relates to a servo control system which can automatically correct a position of a hydrostatically supported member such as a table of a machine tool, rotary shaft or the like.
2. Description of the Prior Art
In general, in order to minutely move a member for the purpose of determining its position or correcting its position, use must be made of a positioning apparatus which can reciprocate the member without backlash, which is not subjected to disturbance, which does not vibrate the member under dynamic loads, which is a large dynamic stiffness and which can control a minute amount of displacement of the member.
The positioning apparatus can reciprocate the member without backlash if the apparatus is operated under the following conditions. That is, a preload of the maximum external load is subjected between a positioning portion of a positioning mechanism and a positioned portion thereof in a driving direction. The amount ot backlash is measured beforehand and the amount of backlash thus measured in advance is compensated when the member is reciprocated as in the case of a NC (numerically-controlled) positioning system. However, in the former case, the load subjected to the positioning mechanism becomes increased by the surplus load to increase the elastic deformation of the positioning mechanism, thereby increasing a positioning error. In the latter case, it is impossible to correct the irregular amount of play due to change in load.
In order to control a minute amount of movement of the movable body, it has been the common practice to move the movable body by minutely deforming a supporting member with the aid of adjustment of a spring force or electromagnetic force, by making use of an elastic deformation of a magnetostrictive element. However, the former apparatus is based on a principle of utilizing the elastic deformation of the supporting member caused by the external force, so that the positioning apparatus is limited in its stiffness and the positioning accuracy under the variable load is also limited.
The latter apparatus is superior in the stiffness of supporting member to the former apparatus, but is always subjected to hysteresis effect induced in the supporting member formed of the magnetostrictive element due to is magnetic property when the element reciprocates.
In order to overcome the above mentioned drawbacks, it has recently been proposed to make use of a hydrostatic bearing for supporting a positioning mechanism. The hydrostatic bearing is provided with two hydrostatic pads arranged at opposite sides of the member or the supporting member and supplied with pressures which are different from each other by a set value, whereby minutely moving the member. Such hydrostatic bearing is relatively high in rigidity and subjected to less hysteresis action, so that it is suitable for minutely moving the member. But, the above-described apparatus constitutes an open loop control mechanism from control technological standpoint of view and hence its positioning accuracy is directly limited by an accuracy of adjusting the pressure supplied to the hydrostatic bearing.
In addition, in order to reduce a deviation of a rotational center of the rotary shaft supported by the hydrostatic bearing, it has been the common practice to work the rotary shaft and the bearing thereof with a high accuracy. The accuracy thus obtained, however, is dependent on the accuracy in machining of the parts, so that the rotatory accuracy is subject to its machining accuracy. In addition, when the rotary shaft is rotated, a dynamic pressure induced in the bearing tends to displace a rotational center of the rotary shaft.
When various kinds of members such as a table of a machine tool are moved along any guide surface, the member tends to lower a motion accuracy with the influence of the accuracy in configuration of the member per se or the guide surface thereof.